Geologic Time

When one mentions "geologic time" to a geoscientist, that person immediately starts to think in terms of millions to billions of years. The Earth organized as a large accreted body between 4.5 and 4.6 billion years ago. The unit of time measurement applied to geologic history remains the year, which probably has had only limited lengthening (with a larger solar orbit) since the beginning. For practical purposes, we continue to work with the standard year of 365.25 days (although the days were once shorter as the Earth rotated faster).

The rocks at the Earth's surface or accessible (e.g., by drilling) in its shallow interior have a variety of ages as one moves from place to place. The question becomes: How does one know at least the approximate age, i.e., how much earlier in the past did this rock or rock unit (e.g., a layer) become initially emplaced (it may be moved by erosion or crustal deformation, etc.)? Two solutions to the question are: 1) determine its relative age and estimate its time of formation by approximation techniques; and 2) use some sort of absolute time dating method.

Relative age determination involves some important rules: 1) the Law of Uniformity (James Hutton):The Present is the Key to the Past; this means that processes we observe in today's world likely operated throughout the past, back even billions of years ago; 2) the Law of Superposition: Under most circumstances when sedimentary layers are deposited in a sequence, the lowest in that sequence was deposited first and is the oldest whereas the highest was last deposited and is the youngest (this may become spatially "untrue" when rocks are displayed by faulting or severe overfolding); 3) the Law of Cross-cutting: any geologic feature (igneous intrusion; fault, etc) that cuts into/across rock units must be younger than these units; 4) the Law of Faunal Succession: in a sequence of rocks of significant thicknesses, there may likely be remains of animals/plants (fossils) that show some systematic evolutionary progression; most commonly the change is towards complexity and diversity and follows morphological and taxonomic modifications.

Until the 18th and 19th Centuries, the age of the Earth was estimated more from biblical interpretations than from scientific determinations. Then, following Hutton, Lyell, Lamarck, Darwin and others two things were realized: 1) it takes thousands to millions of years to deposit layered sedimentary rock units of thicknesses in the 10s to 100s of meters realm; and 2) progressive evolutionary changes also take millions of years for significant advances in the development of genera and species to become recognizable. By the end of the 19th Century, Earth had "aged" from the 4004 BC estimate by Bishop Usher to millions of years. In the 20th Century the age passed two billion years and with more precise methods (radiometric; see below) zeroed in on the present value near 4.6 billion years.

The most indicative method for reaching the billion year age category was simply to measure the thicknesses of units in one place and estimate the time to deposit them (millions of years), then go elsewhere and find at least some of the same units that had different units below or above them, which had to be older or younger. This involves correlation of units based on something in common from place to place, as for example certain animal/plant fossils that are the same or similar, or distinctive rock types in the same sequences. By roaming over a continent and between continents, sequences were recognized, described, and matched with sequences elsewhere that had new member units, within, up, or below. This leads to a general "worldwide" Geologic Column that tries to account for all the deposits laid down during given time intervals (spans) in various locations that can be matched and then expanded by overlapping correlation (see below).

Around the turn of the 18th Century into the 19th, various geologists who were adept at stratigraphic analysis (recognition of layers that had characteristic time markers such as fossils) began to publish their descriptions of the sequences they studied. Others found different sequences and/or some of the same sequences in the descriptions. As the Geologic Column grew, estimates of their ages were made using mainly deposition rates. Individual sequences within the column were assigned times in the past which resulted from the column estimates. It became conventional to give a name to a sequence that seemed to represent a long span of time but with certain diagnostic properties (e.g., a collection of life forms that, while evolving, possessed similarities). These sequences became Periods in a temporal-stratigraphic nomenclature and all rocks contained within the sequence made up a System. Subdivision of Periods into smaller time spans yielded Epochs, with their Rocks being Series. Broader divisions of time made up a some number of Periods (each younger one overlying an older Period - Law of Superposition) were called Eras. Names given to Periods either had some geographic significance (the Cambrian Period was first described and measured in Cambria within the British Isles) or were at the Era Level defined by an appraisal of the dominant life forms and their stages of evolution (the Mesozoic means "Middle" "Life" [zo- is part of a Greek word pertaining to life).

From this approach, a system for expressing geologic time and naming subdivisions has emerged, as shown in this diagram:

This scale has many subdivisions over the last 600,000 million years, since these rocks are well preserved in parts of the world. All rocks older than the beginning of the Paleozoic ("ancient life"), whose oldest period is the Cambrian, are said to be Precambrian - a general term, that is now undergoing further subdivisions. The bulk of geologic time (about 87%) is Precambrian, as shown in this diagram

With the introduction of this nomenclature, we can now look at two figures that use deposition rates, superposition, and correlation to build up a regional Geologic Column covering rocks in Arizona and Utah. The first shows the Column as determined from rock exposed within the Grand Canyon.

Several comments are in order: 1) the horizontal rocks all fall within the Paleozoic Era; since they are horizontal, they rest in the same positions as when deposited - they have not been folded; 2) some rocks within individual Periods are distinct from one another; they constitute Formations (mappable units that are named from where [geographically] they were first described); 2) representatives from some Periods or Epochs are missing - either they were never deposited in the ancient seas that produce the Formations or if deposited have since been eroded away - a fact termed an Unconformity; 4) rocks below the Paleozoic are Precambrian; the upper group are sedimentary but tilted (folded) indicating they were deformed in some type of mountain building, partially eroded and then covered with the lowest Paleozoic units, making an Angular Unconformity (Orogeny; 5) these in turn rest on metamorphic rocks, much older with some upper units having been removed by erosion (reaching into lower levels of the crust) to form a Nonconformity; 6) the metamorphic rocks were intruded by granite, which by the Law of Cross-cutting must be younger; and 7) even now, rocks at the surface (top of the column) are being eroded and probably were undergoing erosion since the Paleozoic; this absebce implies that Mesozoic and Cenozoic rocks are missing, either through erosion or absence of depositing seas; if at some future time seas roll into the region a new unconformity will develop.

The next diagram shows how the Geologic Column has been compiled for this region using correlation:

The link between the Grand Canyon and Bryce columns is the Kaibab Limestone. Bryce contains Mesozoic units, of which the Jurassic Navajo Sandstone is most distinctive. That sandstone unit is near the base of the section at Zion Canyon which preserves Upper (higher) Mesozic Units and an early Tertiary Formation - the Wasatch - in the Cenozoic. So the composite of the three columns or sections has representative deposits in Periods from the Precambrian through the Paleozoic and Mesozoic into the young Cenozoic.

The skills of stratigraphers over the last 200+ years has produced a general geologic column and a time scale that proves to be quite accurate. But this accuracy had to be confirmed by some independent method that measures absolute time. That had to await the discovery of radioactivity in the late 19th Century followed by the realization that radioactive elements (as isotopes) decay (their nucleus is changed to another radionuclide of the same element or more commonly to new elements) at very precise fixed rates. The decay is said to be exponential (for example, a series proceeding as 1-->2-->4-->8-->16-->....). For example, some given amount of the radioactive form of Potassium, K40 will have half its atoms come apart to form radioactive Argon A40 over a long finite time = 1.251 billion years. Consider this diagram:

Half the potassium-40 is gone after 1.251 billion years; half remains. Now if another 1.251 billion years elapses while the mineral containing the potassium (several isotopes), then in 2.502 billion years only 1/4th of the original K-40 will remain and a larger amount of A-40 has developed. The clock on dating begins when the original potassium is incorporated in the rock; argon, a gas, should not be present. Assuming none of the A-40 escapes over time, then a geochronologist need only measure (using a mass spectrometer) the amounts of K-40 and A-40 now present in the rock to set up a ratio and to use the decay rate (given in half-life, the time required for half of any of the radioisotope to decay) to determine the age. If that ratio is K-40/A-40 = 1/8th, the age would be 3753 million years - the time between incorporation of the potassium in the rock (likely, a granite) and the present.

Very accurate ages of incorporation are possible using radiometric dating, provided nothing escape or no element contamination occurs. There are a number of radioactive element that have their own decay schemes. Radioisotopes of Uranium (U) decay at various rates to isotopes of other elements (e.g., Radon) and eventually to isotopes of Lead (Pb). The element Rubidium (found in micas) decays to Strontium. Some elements decay over short half lifes and are confined to dating younger rocks; others are especially suited to determining Precambrian times. If a shale is intruded by a granitic dike (narrow tabular cross-cutting body), the two rock types can be dated by different radioisotope methods, so that the time when each event took place can be fixed fairly accurately.

Thus, since the early 20th Century geoscientists have had a powerful tool to reconstruct when different specific events took place in a complex assemblage of rocks, so that a precise geologic history can unfold. The table below summarizes a generalized history of, mainly, the primitive Earth.

The oldest dated mineral, a zircon from Australia, is age-fixed at 4.1 billion years, but most early ages for rocks fall around 3.6 b.y. Thus most of the Earth's original, and some subsequent, crust has been destroyed (remelted; subducted; broken down by weathering). When oxygen was nearly absent from the atmosphere, the most characteristic rock type was BIF (Banded Iron Formation); its production consuming any oxygen released. As photosynthesis in plants emerged as a working process, oxygen increased, producing iron oxides in the form of Red Beds; then also carbonate rocks became commonplace in sedimentary sequences.

The most important events, from the human perspective, have been the origin and time of appearance of the first living forms, and the subsequent development of the major phylla and orders of life on land and sea. The first indications of life extend now to earlier than 3.5 billion years ago. Early life was single-celled - procaryotic. Multi-celled life - eucaryotic - appeared in the Precambrian. The greatest diversity ("explosion") of life occurred at then close of the Precambrian into the Cambrian Period. Since then at least 6 mass extinctions (significant fraction of all life types at the time) have occurred. The time sequence of life on Earth is depicted in this "cartoon", in which principles of evolution govern the progression and emergence of new phylla:

The above taken from "Life in the Universe", Steven Weinberg, Scientific American, Oct. 1994

Much of the last topics above is examined in more detail on page 20-12.

Plate Tectonics; Mountain Building; Continental Growth/Movements

A good review of these topics which supplements the coverage below has been prepared online by the U.S. Geological Survey.

As the 20th Century began, major unsolved problems in understanding the Earth's geology included the distribution of rock type by age and by structural state, the causes of mountain building and other modes of deformation, the distribution of earthquakes and volcanoes, and the nature of the seafloor's composition and geologic features. This worldwide map shows three fundamental rock units: 1) ancient Precambrian igneous-metamorphic rocks exposed as Shields (red), 2) mostly flat-lying sedimentary rocks (orange), and 3) folded/faulted rocks in mountain belts (brown)

From Tarbuck & Lutgens, The Earth, 3rd ed., 1990

We will concentrate our discussion of Plate Tectonics and continental growth on North America as a definitive example. As radiometric ages were determined for the shield-like rocks within the continents which were either exposed at the surface, underlay the flat rocks, or were within the interior of the mountain belts, patterns of age intervals were determined, as shown in the next two figures. The rocks older than about 500 million years have been called the "basement" - a term that suggests they are found at the "bottom" of the accessible crust. Their age and distribution have been interpreted to mean that the continents had somehow grown (enlarged) around their oldest crustal components (nuclei) by addition of rock assemblages - each being crustal landmasses (terranes) developed elsewhere with their own characteristic age parameters. The additions are mainly through collisions of crustal masses resulting from plate tectonic movements throughout the ancient past; these accreted (or tectonostratigraphic) terranes are discussed in the second half of Section 17. The expanding continental nuclear units coalesce to become part of the "craton" which consists of both exposed (as a "shield") and buried basement rocks (mostly now metamorphic and igneous). Maps of these units are shown in two different versions, with moderate differences in distribution and age among specific added components; the orogenic (mountain-building) units known as the Appalachians and the Cordillera make up the outer parts of North America and are the youngest (less than 500 million years) of the major components of the continent (which lies embedded in the North American Plate). In these maps sedimentary rocks deposited over the past 550 million years that lie between the eastern and western mountain belts in the continental interior have been "removed" so as to reveal the basement units they overlie.

From Lutgens and Tarbuck, The Earth, 3rd Ed.

By the latter half of the 19th Century, studies of the Appalachian Mountains and others led to this general picture of a linear, wide sequence of all three types of rocks that had been deformed and may still show topographic evidence of differential erosion producing present-day mountains.

One part of the mountain system consists of folded/faulted sedimentary rocks. These appear to have been deposited on the other part, deformed segments of the basement, much being Precambrian, both with younger granitic intrusions.

Attempts to explain this mountain structure led to various hypotheses, chief of which were put forth by James Hall and James Dana in the later 1800s from their studies of the Appalachians. In this model, there were two regions near a continental margin that downsank to form Geosynclines -troughs that could receive over time deposits of sediments that exceed 15000 meters (~50000 ft) in accumulated thickness.

From time to time these sediments (converted by burial to sedimentary rocks) would be squeezed by compressional forces causing uplift and folding of mountains over at least part of the length of the linear trough(s) (typically 1000-3000 miles; widths around 500 miles), followed by erosion (unconformities) and renewed deposition. Finally, the entire geosynclinal belt was subjected to intense compression leading to the main phase of orogeny (mountain-building) and general uplift that over time causes removal of some of the mountain units through erosion, in places exposing the basement.

However, as the 20th century progressed and new information about mountains and continents accrued, problems with this model were identified. Alfred Wegener, a European meteorologist, noted that if continental outlines (including submerged edges near shore) for Europe, Africa, North America and South America were placed next to each other (say, by cutting them out like "paper dolls), these continents show a remarkable fit, shown here.

Wegener hypothesized that at some time in the past those continents had been conjoined as a single supercontinent he called Pangaea. Then they broke apart (continental split) and starting moving away from one another until reaching their present positions. This was continuous but is shown here in three steps reprenting stages since breakup began near the end of the Paleozoic. He called the process Continental Drift and speculated that thermal currents in the mantle may provide the driving forces. Some evidence he cited to support the idea includes structural continuity (mountain systems on continental pairs match when the fit restores their original positions, glacial similarites, and, strongest of all, presence of the same animals/plants (as fossils) of types that could not swim across oceans or float far by air.

Pangaea itself further ruptured into two continents - Laurasia (north) and Gondwanaland (south), each of which split further into the present geographic layout. Those interested in the sequences that have now been reconstructed should visit this website produced by the Geology Department at the University of Wisconsin at Green Bay.

By the 1950s the geosynclinal model had been discarded but continental drift remained in favor. A new paradigm was needed. A series of observations led to the general model of Plate Tectonics which became a major revolution in geological thinking about the realities of a dynamic Earth.

The first bits of explanatory evidence came from discoveries about the deeper ocean sea floor. Aerial geophysical flights across stretches of the ocean uncovered an unexpected magnetic phenomenon. Evidence found by magnetic properties analysis of the extruded oceanic basalt (which contains magnetite and other iron minerals that act like "miniature compass needles" that align so as to point to the Pole [arbitrarily called North] where magnetic lines of force in the Earth's magnetic field enter the planet at the time of lava solidification) permits establishment of the polar directions at the time the basalt sample crystallizes. Studies of samples at different distances from the ridge crest found that the North and South Magnetic Poles reverse their polarity (South becomes the entry point for magnetic lines of force and North the exit point) over time intervals of less than a hundred thousand to a few million years (on average every 200000 years, leaving the field at minimum strength over about 3000 years) during the reversal period. When survey flights passed across Mid-Ocean Ridges, patterns like the one below were registered; each stripe indicates that for the time basaltic lava extrudes (at rates of 5 to 20 cm/year) the enclosed magnetic minerals for the full interval needed to produce the width of a stripe (10s to 100+ km) are pointed either to today's magnetization (normal N-S) or to the opposite polarity (reverse N-S).

More about this and other relevant information applicable to Geology has already been reviewed for you in the subsection of the Introduction that dealt with Geophysical Remote Sensing

Of special significance is that the patterns on either side are mirror images of each other. This can be explained by assuming that new ocean crust pours out at the ridge and spread away in both directions over the span of time in which one polarity - normal or reverse is operative. Spreading rates to either side are about equal. The series of normal-reverse polarities alternate over time giving the symmetric pattern observed.

Then, deep sea dredging and later drilling brought up samples of the basaltic ocean crust which could be dated radiometrically. Over the years enough parts of the oceans' floors were reached, sampled, and dated. The general trend, when data points were plotted, was for (magnetized) stripes of basalt to be youngest at the ridges and oldest where ocean floors meet continents. This is the general picture.

From Hamblin, Earth's Dynamic Systems, 6th Ed., 1991

This surprising mechanism of adding new material at ridges and having surficial layers move away from the Mid-Ocean Ridges (found in the Pacific and Indian Oceans too) was independently, and almost simultaneously named by Dr. Harry Hess (Princeton) and Dr. Robert Dietz (NOAA) as Sea Floor Spreading. It started others to thinking about how it works and the consequences applied to the Earth's exterior. As new data from geophysics on earthquake epicenters (surface projections of source areas at depth) and better plots of volcanic activity were shown on maps, this general pattern became obvious (see also the two relevant illustrations on page Intro 2-1c):

From Hamblin, Earth's Dynamic Systems, 6th Ed.

The way to explain these observations now opened fast for geoscientists. No one individual is credited with "thinking up" all the basics of Plate Tectonics; many contributed vital evidence and innovative operational models during a relatively short period in the 1960s onward. The essential idea starts with this assumption in an attempt to explain the earthquake and volcanic distributions: The present-day Earth outer shell is broken into 6 major plates (cover large areas) and some smaller ones. They have several types of boundaries (see below) and are about 200 km thick. (If a large plate could be "plucked" from the Earth it would resemble an orange peel, being curved as a segment of a sphere). The plates consist of a sequence of rock types, either basaltic crust and iron-magnesium upper mantle or continental crust overlying some basaltic crust and mantle, which makes up relatively rigid rocks in the Lithosphere. Below the lithosphere is mantle rock soft enough (through heat) to allow the lithospheric plates to "glide" laterally across parts of the globe. This map shows today's major and minor plates now identified as separate moving bodies; over time in the past and projected into the future, the plates size and location will vary as individual plates grow or are consumed:

Four types of plate boundaries or margins have been recognized:

Boundary type A is diverging; at a Mid-Ocean Ridge, lava extrudes in two directions as it adds to adjacent plates. This is the region of the main driving force that moves plates. Boundary B occurs where two ocean type plates (no nearby continental crust) converge head on. One plate is forced under the other, this is called subduction in which the underthrust plate gradually melts and dissipates (becomes part of the mantle rock) when pushed to increasing depths. The process leads to indentations of the crust that oceanographers call trenches; deepest on Earth today is the Marianas Trench in the Pacific, whose bottom is nearly 35000 feet (10 km) beneath sea surface. The C Boundary refers to a converging margin where continental crust meets continental crust on the second plate. Boundary D is somewhat different - it does not develop at a diverging or converging boundary but is either at a plate edge where two plates slide past along transform faults or is one of a series of transform faults that aid movement within a plate.

We are now ready to define the interaction of plates through this schematic diagram:

Melting of the mantle, mainly in the heated asthenosphere, causes lava to move upwards into a long linear fracture system that builds up as a Mid-Ocean Ridge; the two plates on either side are diverging. To the left one of these ocean plates meets another and subducts. Frictional and residual heat produces magmas on the up plate side that reach the surface as lavas which accumulate into volcanic structures. These produce Island Arcs, constructed around the volcanoes; Indonesia, Japan, and the Aleutians are three examples. To the right, the other plate meets a continent-bearing plate and also subducts. Melting again produces magmas that intrude near the continental margin and surface a volcanic lavas (either flows or volcanoes); the American Cascades are of this nature. Finally, within the continental upwelling convection currents may be forcing the continent to pull apart as a rifting zone which in time may split the continent into two or more parts (Pangaea).

The diagram below ties this type of plate margin into the rock cycle.

From Tarbuck and Lutgens, The Earth, 3rd Ed.

The nature of the driving forces seems to be tied to slow movements something like currents (analogy: in a boiling pot of water) of very hot, plasticlike mantle rock. These involve heat transfer by convection. Some evidence suggests these convection currents (shown below) originate near the mantle/core boundary. Other signs indicate shallower origins or perhaps a secondary set of currents in the upper mantle only.

Just to emphasize the characteristics of the plate tectonics model, this is the third variant we have shown on these two pages. The upper diagram follows the full mantle convection hypothesis; other versions show the cycle to have the major flow to be sea floor spreading in the upper half and flow of the upper mantle towards the ridge exits within or below the level of the asthenosphere.

From McGeary and Plummer: Physical Geology -Earth Revealed, 1992

So, how does the Plate Tectonic Model tie in with the notion of Continental Drift? Or, more to the point, what is the evidence for drift? The chief proof comes from Polar Wandering. At the time rocks containing magnetite solidify on the continent, the magnetic grains act like tiny magnets and point to the north pole as does the needle on a compass. Assuming that the magnetic poles remain constant in position (but not in polarity) over vast time periods - for which there is good evidence, these grains serve as markers suited to locating the pole at the time they were encased in cooled rock (usually basalts). If the polarity is determined in rocks of different ages, the positions can be plotted, as follows:

From Tarbuck and Lutgens, The Earth, 3rd Ed., 1990

This resulting Polar Wandering plot is explained as follows: On the left diagram are a pair of curves made by connecting the geographic location of the pole in North America and in Europe yielding points at different times - the progression is from 300 million years to the Present. Note that the two curves do not fit; each was constructed from pole position data acquired on North America alone and Europe alone. On the right diagram, in which the continents have pushed together as they are claimed by Continental Drift to have been prior to breakup of Pangaea, the two curves now coincide. This is convincing proof that at that time the continents were conjoined.

We have already alluded to the possibility (actually it is common) of two plates each bearing a large landmass (up to continental size) colliding. If little oceanic crust is involved at later stages, the continents will collide head on, will probably weld to each other, and one may override the other, with the result that the now combined continents in the collision zone actually thicken. This has happened in the case of the Indian subcontinent heading into the "underbelly" of Central Asia, as sequenced in this diagram. The result is the Himalayan Mountains, highest on Earth.

This process suggests one means by which continents (which contain more silica-rich rocks like granites and usually have extensive sedimentary rock cover) can grow in size. The various plates in modern times have not only continents on them but many smaller features such as island arcs, ruptured continental fragments, and even spreading ridges are within a plate. When collisions occur, some of this "flotsam" may subduct but commonly it is shoved on and welded to the continental margin. These additions are called Terranes and the assemblage of individual terranes that arrived at separate times make up what is term Accreted Terranes (see Section 17). The western edge of North America has been built up by a succession of accreted terranes, as indicated in the next two figures:

From Skinner and Porter, Physical Geology, 1987

The eastern part of North America also has a large number of terranes added both before and after the Pangaea split. Continents may in fact build up largely by terrane accretion, as suggested in the time map of Provinces in North America appearing earlier on this page.

Finally, we want to indicate in some detail how mountain belts are produced. We will look at the Appalachians - the group that led to early ideas of mountain-building in the 1800s. In this five panel illustration, the major steps and events are shown from Precambrian (top) to the present (bottom).

From Skinner & Porter, The Dynamic Earth, 2nd Ed., 1995

This sequence is likely: 1) In Late Precambrian, a plate east of the North American block (already existing for a long time) subducted, causing am island arc and a back arc depositional basin; 2) In Cambrian times, a rupture near the margin produced a second subduction zone pointing in a direction opposing the first; volcanism and deposition continued; 3) In the Ordovician and again in the Devonian, more mini-subduction occurred producing the Taconic and Acadian mountains - precursors to the present; 4) As the main westward movement of the African plate continued to subduct under North America, the African continent itself approached; the Iapetus Ocean between the two continents progressively closed; 5) in time the African continet crashed against North America, closing the Iapetus, but by Triassic times the two continents split and the Atlantic Ocean opened; there no longer is an active subductive zone next to either continent. This running description helps to demonstrate that plate tectonic action can lead to some complicated sequences of events.

So there you have it: In these two pages some of the rudiments of Geology - some but not all its principal ideas - have been reviewed to give you a working background to appreciate material in this and other Sections. To learn more - hunt through the Internet or take a course in college. And, a plug given here to reach any who, like the writer (NMS), may aspire to Geology as a career: I point with the following photo to a "trademark" of this profession - the joy of being outdoors doing field work, as illustrated in this trip in 1966 which I participated in during an Oregon Conference on Volcanism: